Method for making a lithographic printing plate according to the silver salt diffusion transfer process

ABSTRACT

The present invention provides a method for making a lithographic printing plate according to the silver salt diffusion transfer process comprising the steps of image-wise exposing an imaging element comprising on a support a photosensitive layer comprising a silver halide emulsion and an image receiving layer containing physical development nuclei and developing said imaging element in the presence of developing agent(s) and silver halide solvent(s) using an alkaline processing liquid characterized in that said development is carried out in the presence of a meso-ionic compound. The present invention further provides an imaging element and alkaline processing liquid for use in said method.

This is a continuation of application Ser. No. 07/986,071 filed Dec. 4, 1992, now abandoned, which is a continuing application of patent applications Ser. No. 07/810,146 filed Dec. 19, 1991, now abandoned, Ser. No. 07/810,150 filed Dec. 19, 1991, now U.S. Pat. No. 5,200,294, and Ser. No. 07/810,151 filed Dec. 19, 1991, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a method for making a lithographic printing plate according to the silver salt diffusion transfer process.

BACKGROUND OF THE INVENTION

The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).

In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.

A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. EP-A-423399 and EP-A-410500.

The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.

As for other printing plates it is required that the printing plates obtained according to the DTR-process have a high printing endurance, good ink acceptance in the printing areas and no ink acceptance in the non-printing areas (so called toning) during start-up of the printing process is limited. With respect to these requirements the state of the transferred silver i.e. the silver formed in the image-receiving layer plays a very important role. Parameters that are known to control the state of the transferred silver are e.g. the rate of development of the exposed silver halide (chemical development), rate of dissolution of the non-exposed silver halide by the silver halide solvent(s), rate of diffusion of the silver halide complexes, rate of development of the silver halide complexes in the image receiving layer (physical development) etc.. Although other factors such as for example the plate construction have an influence on the aforementioned parameters the type of silver halide solvent influences most of the mentioned parameters to a large extent.

A further parameter for improving the printing properties of the printing plate especially printing endurance and staining is the differentiation between the hydrophobicity of the silver image and the hydrophilic background. To improve the hydrophobicity hydrophobizing agents are usually added to one or more processing liquids used for preparing the lithographic printing plate. However when these agents are used in large amounts heavy toning of the printing plate occurs.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for making a lithographic printing plate according to the DTR-process having good printing properties i.e. good ink acceptance in the printing areas, no ink acceptance in the non-printing areas and a high printing endurance.

Further objects of the present invention will become clear from the description hereinafter.

According to the present invention there is provided a method for making a lithographic printing plate according to the silver salt diffusion transfer process comprising the steps of image-wise exposing an imaging element comprising on a support a photosensitive layer comprising a silver halide emulsion and an image receiving layer containing physical development nuclei and developing said imaging element in the presence of developing agent(s) and silver halide solvent(s) using an alkaline processing liquid characterized in that said development is carried out in the presence of a meso-ionic compound.

According to the present invention there is also provided an imaging element and an alkaline processing liquid for use in the above defined method.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

According to the present invention it has been found that a lithographic printing plate prepared according to the DTR-process in the presence of a meso-ionic compound has an increased printing endurance, good ink acceptance at the start of the printing process and no or little staining or toning of the plate occurs.

Meso-ionic compounds as referred to in the present invention are a group of compounds defined by W. Baker and W. D. Ollis as "5- or 6-membered heterocyclic compounds which cannot be represented satisfactorily by any one covalent or polar structure and possesses a sextet of π-electrons in association with the atoms comprising the ring. The ring bears a fractional positive charge balanced by a corresponding negative charge located on a covalently attached atom or group of atoms" as described in Quart. Rev., Vol. 11, p. 15 (1957) and Advances in Heterocyclic Chemistry, Vol. 19, P. 4 (1976).

Preferred meso-ionic compounds are those represented by formula (I): ##STR1## wherein M represents a 5- or 6-membered heterocyclic ring composed of at least one member selected from the group consisting of a carbon atom, an oxygen atom, a sulfur atom and a selenium atom; and A.sup.θ represents --O.sup.θ, --S.sup.θ or --N.sup.θ --R, wherein R represents an alkyl group (preferably having 1 to 6 carbon atoms), a cycloalkyl group (preferably having 3 to 6 carbon atoms), an alkenyl group (preferably having 2 to 6 carbon atoms) an alkynyl group (preferably having 2 to 6 carbon atoms), an aralkyl group, an aryl group (preferably having 6 to 12 carbon atoms), or a heterocyclic group (preferably having 1 to 6 carbon atoms).

In formula (I), examples of the 5-membered heterocyclic ring as represented by M include an imidazolium ring, a pyrazolium ring, an oxazolium ring, an isoxazolium ring, a thiazolium ring, an isothiazolium ring, a 1,3-dithiol ring, a 1,3,4- or 1,2,3 oxadiazolium ring, a 1,3,2-oxathiazolium ring, a 1,2,3-triazolium ring, a 1,3,4-triazolium ring, a 1,3,4-, 1,2,3- or 1,2,4-thiadiazolium ring, a 1,2,3,4-oxatriazolium ring, a 1,2,3,4-tetrazolium ring and a 1,2,3,4-thiatriazolium ring.

Meso-ionic compounds are known for use in the fixing step of a photographic process as disclosed in EP-A-431568. Triazolium thiolate meso-ionic compounds are well known in silver halide photography and are used e.g. for increasing temperature latitude as disclosed in JP-A-60-117240, for reducing fog as disclosed in U.S. Pat No. 4,615,970, in preparing silver halide emulsions as disclosed in U.S. Pat. No. 4,631,253, in bleach etching baths as disclosed in EP-A-321839, to prevent pressure marks as disclosed in U.S. Pat. No. 4,624,913 etc..

However from these disclosures it could not be expected that the printing properties and especially the printing endurance could be increased by using such compounds during preparation of a lithographic printing plate according to the DTR-process.

Preferred meso-ionic compounds for use in accordance with the present invention are triazolium thiolares and more preferably 1,2,4-triazolium-3-thiolates and most preferably those that correspond to the following formula: ##STR2## wherein R⁷ and R⁸ each independently represents an unsubstituted or substituted alkyl group, alkenyl group, cycloalkyl group, aralkyl group, aryl group or heterocyclic group, A represents an unsubstituted or substituted alkyl group, alkenyl group, cycloalkyl group, aralkyl group, aryl group, heterocyclic group or --NR⁹ R¹⁰ and R⁹ and R¹⁰ each independently represents hydrogen, an alkyl group or aryl group and wherein R⁷ and R⁸ or R⁸ and A or R⁹ and R¹⁰ can combine with each other to form a 5- or 6-membered ring.

Specific examples of 1,2,4-triazolium-3-thiolates suitable for use in accordance with the present invention are shown in table 1.

                  TABLE 1                                                          ______________________________________                                          ##STR3##                      (1)                                              ##STR4##                      (2)                                              ##STR5##                      (3)                                             ______________________________________                                    

According to the preferred embodiment of the present invention at least part and most preferably all of the meso-ionic compound is preferably present in the alkaline processing liquid used for developing the image-wise exposed imaging element. Preferably the amount of meso-ionic compound in the alkaline processing liquid is between 0.1 mmol/l and 25 mmol/l and more preferably between 0.5 mmol/l and 10 mmol/l and most preferably between 1 mmol/l and 5 mmol/l.

However the meso-ionic compound may be incorporated in one or more layers comprised on the support of the imaging element. This offers the advantage of replenishment of said meso-ionic compound in the alkaline processing liquid. Preferably the meso-ionic compound is incorporated in a layer of the imaging element that does not contain silver halide such as e.g. in a backing layer, in the image receiving layer, a subbing layer etc.. The meso-ionic compound is preferably contained in the imaging element in a total amount between 0.1 and 10 mmol/m², more preferably between 0.1 and 0.5 mmol/m² and most preferably between 0.5 and 1.5 mmol/m².

According to the present invention the alkaline processing liquid used for developing the imaging element preferably contains a silver halide solvent although this is no absolute requirement since meso-ionic compounds can be used as the sole silver halide solvent. Preferably the silver halide solvent is used in an amount between 0.05% by weight and 5% by weight and more preferably between 0.05% by weight and 2% by weight. Suitable silver halide solvents for use in connection with the present invention are e.g. 2-mercaptobenzoic acid, cyclic imides, oxazolidones, thiocyanates and thiosulfates. Further silver halide solvents that can be used in connection with the present invention are alkanolamines. Alkanolamines that are suitable for use in accordance with the present invention may be of the tertiary, secondary or primary type. Examples of alkanolamines that may be used in connection with the present invention correspond to the following formula: ##STR6## wherein X and X' independently represent hydrogen, a hydroxyl group or an amino group, 1 and m represent 0 or integers of 1 or more and n represents an integer of 1 or more. Preferably used alkanolamines are e.g. N-(2-aminoethyl)ethanolamine, diethanolamine, N-methylethanolamine, triethanolamine, N-ethyldiethanolamine, diisopropanolamine, ethanolamine, 4-aminobutanol, N,N-dimethylethanolamine, 3-aminopropanol, N,N-ethyl-2,2'-iminodiethanol etc. or mixtures thereof.

According to the present invention the alkanolamines are preferably present in the alkaline processing liquid in a concentration preferably between 0.1% and 5% by weight. However part or all of the alkanolamine can be present in one or more layers of the imaging element.

Especially good results are obtained when a meso-ionic compound, preferably a triazolium thiolate, is used in combination with a thioether as silver halide solvent. The best results are obtained when a combination of a meso-ionic compound, a thioether and alkanol amine are used as silver halide solvents. Preferably used thioethers correspond to the following general formula:

    Z--(R.sup.1 --S).sub.t --R.sup.2 --S--R.sup.3 --Y

wherein Z and Y each independently represents hydrogen, an alkyl group, an amino group, an ammonium group, a hydroxyl, a sulfo group, a carboxyl, an aminocarbonyl or an aminosulfonyl, R¹ R² and R³ each independently represents an alkylene that may be substituted and optionally contain a oxygen bridge and t represents an integer from 0 to 10.

Examples of thioether compounds corresponding to the above formula are disclosed in e.g. U.S. Pat. No. 4,960,683 and specific examples are listed in table 2

                  TABLE 2                                                          ______________________________________                                         (HOCH.sub.2CH.sub.2).sub.2 S                                                                               1                                                  HOCH.sub.2CH.sub.2SCH.sub.2SCH.sub.2CH.sub.2OH                                                             2                                                  HOCH.sub.2CH.sub.2S(CH.sub.2).sub.2SCH.sub.2CH.sub.2OH                                                     3                                                  HOCH.sub.2CH.sub.2S(CH.sub.2).sub.3SCH.sub.2CH.sub.2OH                                                     4                                                  HOCH.sub.2CH.sub.2S(CH.sub.2).sub.4SCH.sub.2CH.sub.2OH                                                     5                                                  C.sub.2 H.sub.5SCH.sub.2CH.sub.2SCH.sub.2CHOHCH.sub.2OH                                                    6                                                  (HOCH.sub.2CH.sub.2SCH.sub.2CH.sub.2).sub.2 O                                                              7                                                  HOCH.sub.2CH.sub.2SCH.sub.2CH.sub.2N(CH.sub.3).sub.2                                                       8                                                  (HOCH.sub.2CHOHCH.sub.2SCH.sub.2).sub.2                                                                    9                                                  HOOCCH.sub.2SCH.sub.2SCH.sub.2COOH                                                                         10                                                 HOOCCH.sub.2S(CH.sub.2).sub.2SCH.sub.2COOH                                                                 11                                                 HOOCCH.sub.2S(CH.sub.2).sub.3SCH.sub.2COOH                                                                 12                                                 HOOC(CH.sub.2).sub.2SCH.sub.2S(CH.sub.2).sub.2COOH                                                         13                                                 HOOC(CH.sub.2).sub.2S(CH.sub.2).sub.2S(CH.sub.2).sub.2COOH                                                 14                                                 HOOC(CH.sub.2).sub.2S(CH.sub.2).sub.3S(CH.sub.2).sub.2COOH                                                 15                                                  ##STR7##                   16                                                 ______________________________________                                    

According to the present invention the alkaline processing liquid preferably also contains hydrophobizing agents for improving the hydrophobicity of the silver image obtained in the image receiving layer. The hydrophobizing agents used in connection with the present invention are compounds that are capable of reacting with silver or silver ions and that are hydrophobic i.e. insoluble in water or only slightly soluble in water. Generally these compounds contain a mercapto group or thiolate group and one or more hydrophobic substituents e.g. an alkyl group containing at least 3 carbon atoms. Examples of hydrophobizing agents for use in accordance with the present invention are e.g. those described in U.S. Pat. No. 3,776,728, and U.S. Pat. No. 4,563,410. Preferred compounds correspond to one of the following formulas: ##STR8## wherein R⁵ represents hydrogen or an acyl group, R⁴ represents alkyl, aryl or aralkyl. Most preferably used compounds are compounds according to one of the above formulas wherein R⁴ represents an alkyl containing 3 to 16 C-atoms.

According to the present invention the hydrophobizing agents are contained in the alkaline processing liquid in an amount of at least 0.1 g/l, more preferably at least 0.2 g/l and most preferably at least 0.3 g/l. The maximum amount of hydrophobizing agents will be determined by the type of hydrophobizing agent, type and amount of silver halide solvents etc.. Typically the concentration of hydrophobizing agent is preferably not more than 1.5 g/l and more preferably not more than 1 g/l. It is a particular advantage of the present invention that hydrophobizing agents can be used in large concentrations so that the hydrophobicity of the silver image can be improved without substantially increasing staining and toning of the printing plate.

The alkaline processing liquid used in accordance with the present invention preferably has a pH between 9 and 14 and more preferably between 10 and 13. Said pH may be established by an organic or inorganic alkaline substance or a combination thereof. Suitable inorganic alkaline substances are e.g. potassium or sodium hydroxide, carbonate, phosphate etc.. Suitable organic alkaline substances are e.g. alkanolamines. In the latter case the alkanolamines will provide or help maintain the pH and serve as a silver halide complexing agent.

The alkaline processing liquid may also contain the developing agent(s) used in accordance with the present invention. In this case the alkaline processing liquid is called a developer. On the other hand some or all of the developing agent(s) may be present in one or more layers of the imaging element. When all of the developing agents are contained in the imaging element the alkaline processing liquid is called an activator or activating liquid.

Silver halide developing agents for use in accordance with the present invention are preferably of the p-dihydroxybenzene type, e.g. hydroquinone, methylhydroquinone or chlorohydroquinone, preferably in combination with an auxiliary developing agent being a 1-phenyl-3-pyrazolidinone-type developing agent and/or p-monomethylaminophenol. Particularly useful auxiliary developing agents are of the phenidone type e.g. 1-phenyl-3-pyrazolidinone, 1-phenyl-4-monomethyl-3-pyrazolidinone, and 1-phenyl-4,4-dimethyl-3-pyrazolidinone. However other developing agents can be used.

The alkaline processing liquid preferably also contains a preserving agent having antioxidation activity, e.g. sulphite ions provided e.g. by sodium or potassium sulphite. For example, the aqueous alkaline solution comprises sodium sulphite in an amount ranging from 0.15 to 1.0 mol/l. Further may be present a thickening agent, e.g. hydroxyethylcellulose and carboxymethylcellulose, fog inhibiting agents, e.g. potassium bromide, potassium iodide and a benzotriazole which is known to improve the printing endurance, calcium-sequestering compounds, anti-sludge agents, and hardeners including latent hardeners.

Development acceleration can be accomplished with the aid of various compounds to the alkaline processing liquid and/or one or more layers of the photographic element, preferably polyalkylene derivatives having a molecular weight of at least 400 such as those described in e.g. U.S. Pat. Nos. 3,038,805 - 4,038,075 - 4,292,400 - 4,975,354.

According to the method of the present invention for obtaining a lithographic printing plate an imaging element comprising on a support a silver halide emulsion layer and an image receiving layer is image-wise or information-wise exposed and subsequently developed in the presence of a meso-ionic compound using an alkaline processing liquid e.g. as described above.

The development step is preferably followed by a neutralization of the surface of the imaged element by guiding the element through a neutralization liquid having a pH between 5 and 7. The neutralization liquid preferably contains a buffer e.g. a phosphate buffer, a citrate buffer or mixture thereof. The neutralization solution can further contain bactericides, e.g. phenol, thymol or 5-bromo-5-nitro-1,3-dioxan as described in EP 0,150,517. The liquid can also contain substances which influence the hydrophobic/hydrophilic balance of the printing plate obtained after processing of the DTR element, e.g. silica. Further the neutralization solution can contain wetting agents, preferably compounds containing perfluorinated alkyl groups. By adding an inorganic fluoride e.g. NaF in amounts of 80 mg/l or less to the neutralization solution toning of the printing plate can be reduced or avoided. Instead of or in combination with the inorganic fluoride a cationic polymer or a polymer containing amino groups can be added to the neutralization solution for reducing toning of the plate. Specific examples of polymers containing amino groups are e.g. a poly(N-3-(2-aminoethyl)-aminopropylsiloxane) or a dextran comprising a group containing an amino group e.g. Dormacid that is commercially available from Pfeifer & Langen and is the commercial name for a dextran containing diethylaminoethyl groups. These polymers are preferably added to the neutralization solution in amounts of 0.01% to 5% and more preferably in amounts of 0.05% to 1% and most preferably in amounts between 0.05% and 0.5%.

According to the present invention the imaging element can be information-wise exposed in an apparatus according to its particular application, e.g. a conventional process camera containing a conventional light source or a laser containing device.

The imaging element in connection with the present invention for preparing a lithographic printing plate essentially contains on a support a photosensitive layer containing silver halide and an image receiving layer containing physical development nuclei. As described above one of the layers contained on the support of the imaging element may contain a meso-ionic compound.

Supports suitable for use in accordance with the present invention may be opaque or transparent, e.g. a paper support or resin support. When a paper support is used preference is given to one coated at one or both sides with an Alpha-olefin polymer, e.g. a polyethylene layer which optionally contains an anti-halation dye or pigment. It is also possible to use an organic resin support e.g. cellulose nitrate film, cellulose acetate film, poly(vinyl acetal) film, polystyrene film, poly(ethylene terephthalate) film, polycarbonate film, polyvinylchloride film or poly-Alpha-olefin films such as polyethylene or polypropylene film. The thickness of such organic resin film is preferably comprised between 0.07 and 0.35 mm. These organic resin supports are preferably coated with a hydrophilic adhesion layer which can contain water insoluble particles such as silica or titanium dioxide. Metal supports e.g. aluminium may also be used in accordance with the present invention.

The image receiving layer containing physical development nuclei used in accordance with the present invention is preferably free of hydrophilic binder but may comprise small amounts up to 30% by weight of the total weight of said layer of a hydrophilic colloid e.g. polyvinyl alcohol to improve the hydrophilicity of the surface. Preferred development nuclei for use in accordance with the present invention are sulphides of heavy metals e.g. sulphides of antimony, bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver, and zinc. Especially suitable development nuclei in connection with the present invention are palladium sulphide nuclei. Other suitable development nuclei are salts such as e.g. selenides, polyselenides, polysulphides, mercaptans, and tin (II) halides. Heavy metals, preferably silver, gold, platinum, palladium, and mercury can be used in colloidal form.

The photographic silver halide emulsion(s) used in accordance with the present invention can be prepared from soluble silver salts and soluble halides according to different methods as described e.g. by P. Glafkides in "Chimie et Physique Photographique", Paul Montel, Paris (1967), by G. F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London (1966), and by V. L. Zelikman et al in "Making and Coating Photographic Emulsion", The Focal Press, London (1966).

The photographic silver halide emulsions used according to the present invention can be prepared by mixing the halide and silver solutions in partially or fully controlled conditions of temperature, concentrations, sequence of addition, and rates of addition. The silver halide can be precipitated according to the single-jet method or the double-jet method.

The silver halide particles of the photographic emulsions used according to the present invention may have a regular crystalline form such as a cubic or octahedral form or they may have a transition form. They may also have an irregular crystalline form such as a spherical form or a tabular form, or may otherwise have a composite crystal form comprising a mixture of said regular and irregular crystalline forms.

According to the present invention the emulsion or emulsions preferably consist principally of silver chloride while a fraction of silver bromide is present ranging from 1 mole % to 40 mole %. The emulsions preferably belong to the core/shell type well known to those skilled in the art in the sense that substantially all the bromide is concentrated in the core. This core contains preferably 10 to 40% of the total silver halide precipitated, while the shell consists preferably of 60 to 90% of the total silver halide precipitated.

The average size of the silver halide grains may range from 0.10 to 0.70 μm , preferably from 0.25 to 0.45 μm.

The size distribution of the silver halide particles of the photographic emulsions to be used according to the present invention can be homodisperse or heterodisperse. A homodisperse size distribution is obtained when 95% of the grains have a size that does not deviate more than 30% from the average grain size.

Preferably during the precipitation stage Iridium and/or Rhodium containing compounds or a mixture of both are added. The concentration of these added compounds ranges from 10⁻⁸ to 10⁻³ mole per mole of AgNO₃, preferably between 10⁻⁷ and 10⁻⁶ mole per mole of AgNO₃. This results in the building in in the silver halide crystal lattice of minor amounts of Iridium and/or Rhodium, so-called Iridium and/or Rhodium dopants. As known to those skilled in the art numerous scientific and patent publications disclose the addition of Iridium or Rhodium containing compounds or compounds containing other elements of Group VIII of the Periodic System during emulsion preparation.

The emulsions can be chemically sensitized e.g. by adding sulphur-containing compounds during the chemical ripening stage e.g., allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Also reducing agents e.g. the tin compounds described in BE-P 493,464 and 568,687, and polyamines such as diethylene triamine or derivatives of aminomethane-sulphonic acid can be used as chemical sensitizers. Other suitable chemical sensitizers are noble metals and noble metal compounds such as gold, platinum, palladium, iridium, ruthenium and rhodium. This method of chemical sensitization has been described in the article of R. KOSLOWSKY, Z, Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

The emulsions of the DTR element can be spectrally sensitized according to the spectral emission of the exposure source for which the DTR element is designed.

Suitable sensitizing dyes for the visible spectral region include methine dyes such as those described by F. M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons. Dyes that can be used for this purpose include cyanine dyes, merocyanine dyes, complex cyanine dyes, complex merocyanine dyes, homopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes are those belonging to the cyanine dyes, merocyanine dyes, complex merocyanine dyes.

In the case of a conventional light source, e.g. tungsten light, a green sensitizing dye is needed. A preferred green sensitizing dye in connection with the present invention is represented by the following chemical formula: ##STR9##

In case of exposure by a red light emitting source, e.g. a LED or a HeNe laser a red sensitizing dye is used. A preferred red sensitizing dye is: ##STR10##

In case of exposure by an argon ion laser a blue sensizing dye is incorporated. A preferred blue sensitizing dye is represented by: ##STR11##

In case of exposure by a semiconductor laser special spectral sensitizing dyes suited for the near infra-red are required. Suitable infra-red sensitizing dyes are disclosed in i.a. U.S. Pat. Nos 2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921, 3,582,344, 3,623,881 and 3,695,888. A preferred infra-red sensitizing dye is: ##STR12##

To enhance the sensitivity in the near infra-red region use can be made of so-called supersensitizers in combination with infra-red sensitizing dyes. Suitable supersensitizers are described in Research Disclosure Vol 289, May 1988, item 28952. A preferred supersensitizer is Compound IV of the Disclosure having following formula: ##STR13##

The spectral sensitizers can be added to the photographic emulsions in the form of an aqueous solution, a solution in an organic solvent or in the form of a dispersion.

The silver halide emulsions may contain the usual stabilizers e.g. homopolar or salt-like compounds of mercury with aromatic of heterocyclic rings such as mercaptotriazoles, simple mercury salts, sulphonium mercury double salts and other mercury compounds. Other suitable stabilizers are azaindenes, preferably tetra- or penta-azaindenes, especially those substituted with hydroxy or amine groups. Compounds of this kind have been described by BIRR in Z. Wiss. Photogr, Photophys. Photochem, 47, 2-27 (1952). Other suitable stabilizers are i.a. heterocyclic mercapto compounds e.g. phenylmercaptotetrazole, quaternary benzothiazole derivatives, and benzotriazole. Preferred compounds are mercapto substituted pyrimidine derivatives as disclosed in U.S. Pat. No. 3,692,527.

The silver halide emulsions may contain pH controlling ingredients. Preferably the emulsion layer is coated at a pH value below the isoelectric point of the gelatin to improve the stability characteristics of the coated layer. Other ingredients such as antifogging agents, development accelerators, wetting agents, and hardening agents for gelatin may be present. The silver halide emulsion layer may comprise light-screening dyes that absorb scattering light and thus promote the image sharpness. Suitable light-absorbing dyes are described in i.a. U.S. Pat. No. 4,092,168, U.S. Pat. No. 4,311,787 and DE-P 2,453,217.

In an especially preferred embodiment the emulsion layer contained in the imaging element contains a compound which comprises in its molecular structure a group capable of adsorbing to silver halide and a group capable of reducing silver halide. Compounds of this kind have been disclosed in EP-A-449340. In this way a combination of a stabilizing and a development activating function in one compound is achieved. A preferred compound belonging to this class is represented by the following formula: ##STR14##

More details about the composition, preparation and coating of silver halide emulsions can be found in e.g. Product Licensing Index, Vol. 92, December 1971, publication 9232, p. 107-109.

In addition to the above described emulsion layer and image receiving layer other hydrophilic colloid layers in water permeable relationship with these layers may be present. For example it is especially advantageous to include a base-layer between the support and the photosensitive silver halide emulsion layer. In a preferred embodiment of the present invention said base-layer serves as an antihalation layer. This layer can therefore contain the same light-absorbing dyes as described above for the emulsion layer; as alternative finely divided carbon black can be used for the same antihalation purposes as described in U.S. Pat. No. 2,327,828. On the other hand, in order to gain sensitivety, light reflecting pigments, e.g. titaniumdioxide can be present. Further this layer can contain hardening agents, matting agents, e.g. silica particles, and wetting agents. At least part of these matting agents and/or light reflection pigments may also be present in the silver halide emulsion layer the most part however preferably being present in said base-layer. As a further alternative the light reflecting pigments may be present in a separate layer provided between the antihalation layer and the photosensitive silver halide emulsion layer.

In a preferred embodiment in connection with the present invention a backing layer is provided at the non-light sensitive side of the support. This layer which can serve as anti-curl layer can contain i.a. matting agents e.g. silica particles, lubricants, antistatic agents, light absorbing dyes, opacifying agents, e.g. titanium oxide and the usual ingredients like hardeners and wetting agents. The backing layer can consist of one single layer or a double layer pack.

The hydrophilic layers usually contain gelatin as hydrophilic colloid binder. Mixtures of different gelatins with different viscosities can be used to adjust the rheological properties of the layer. Like the emulsion layer the other hydrophilic layers are coated preferably at a pH value below the isoelectric point of the gelatin. But instead of or together with gelatin, use can be made of one or more other natural and/or synthetic hydrophilic colloids, e.g. albumin, casein, zein, polyvinyl alcohol, alginic acids or salts thereof, cellulose derivatives such as carboxymethyl cellulose, modified gelatin, e.g. phthaloyl gelatin etc.

The hydrophilic layers of the photographic element, especially when the binder used is gelatin, can be hardened with appropriate hardening agents such as those of the epoxide type, those of the ethylenimine type, those of the vinylsulfone type e.g. 1,3-vinylsulphonyl-2-propanol, methylenebis(sulfonylethylene), chromium salts e.g. chromium acetate and chromium alum, aldehydes e.g. formaldehyde, glyoxal, and glutaraldehyde, N-methylol compounds e.g. dimethylolurea and methyloldimethylhydantoin, dioxan derivatives e.g. 2,3-dihydroxy-dioxan, active vinyl compounds e.g. 1,3,5-triacryloyl-hexahydro-s-triazine, active halogen compounds e.g. 2,4-dichloro-6-hydroxy-s-triazine, and mucohalogenic acids e.g. mucochloric acid and mucophenoxychloric acid. These hardeners can be used alone or in combination. The binders can also be hardened with fast-reacting hardeners such as carbamoylpyridinium salts of the type, described in U.S. Pat. No. 4,063,952.

Preferably used hardening agents are of the aldehyde type. The hardening agents can be used in wide concentration range but are preferably used in an amount of 4% to 7% of the hydrophilic colloid. Different amounts of hardener can be used in the different layers of the imaging element or the hardening of one layer may be adjusted by the diffusion of a hardener from another layer.

The imaging element used according to the present invention may further comprise various kinds of surface-active agents in the photographic emulsion layer or in at least one other hydrophilic colloid layer. Suitable surface-active agents include non-ionic agents such as saponins, alkylene oxides e.g. polyethylene glycol, polyethylene glycol/polypropylene glycol condensation products, polyethylene glycol alkyl ethers or polyethylene glycol alkylaryl ethers, polyethylene glycol esters, polyethylene glycol sorbitan esters, polyalkylene glycol alkylamines or alkylamides, silicone-polyethylene oxide adducts, glycidol derivatives, fatty acid esters of polyhydric alcohols and alkyl esters of saccharides; anionic agents comprising an acid group such as a carboxy, sulpho, phospho, sulphuric or phosphoric ester group; ampholytic agents such as aminoacids, aminoalkyl sulphonic acids, aminoalkyl sulphates or phosphates, alkyl betaines, and amine-N-oxides; and cationic agents such as alkylamine salts, aliphatic, aromatic, or heterocyclic quaternary ammonium salts, aliphatic or heterocyclic ring-containing phosphonium or sulphonium salts. Preferably compounds containing perfluorinated alkyl groups are used. Such surface-active agents can be used for various purposes e.g. as coating aids, as compounds preventing electric charges, as compounds improving slidability, as compounds facilitating dispersive emulsification and as compounds preventing or reducing adhesion.

The photographic material of the present invention may further comprise various other additives such as e.g. compounds improving the dimensional stability of the photographic element, UV-absorbers, spacing agents and plasticizers.

Suitable additives for improving the dimensional stability of the photographic element are e.g. dispersions of a water-soluble or hardly soluble synthetic polymer e.g. polymers of alkyl (meth)acrylates, alkoxy(meth)acrylates, glycidyl (meth)acrylates, (meth)acrylamides, vinyl esters, acrylonitriles, olefins, and styrenes, or copolymers of the above with acrylic acids, methacrylic acids, Alpha-Beta-unsaturated dicarboxylic acids, hydroxyalkyl (meth)acrylates, sulphoalkyl (meth)acrylates, and styrede sulphonic acids.

According to another embodiment of the present invention a lithographic printing plate can be obtained by means of the DTR-process using an imaging element comprising in the order given a grained and anodized aluminium support, an optional layer of physical development nuclei and a silver halide emulsion layer. The imaging element of the present embodiment may be imaged using a camera-exposure or a scanning exposure as described above followed by a development step in the presence of development agent(s) and silver halide solvent(s) according to the invention so that a silver image is formed in the physical development nuclei layer or directly on the aluminium support. Subsequently the silver halide emulsion layer and any other optional hydrophilic layers are removed by rinsing the imaged element with water so that the silver image is exposed. Finally the hydrophobic character of the silver image is preferably improved using a finishing liquid comprising hydrophobizing agents as described above.

The present invention will now be illustrated by the following examples without however limiting it thereto. All parts are by weight unless otherwise specified.

EXAMPLE 1 Preparation of the silver halide emulsion coating solution

A silver chlorobromide emulsion composed of 98.2 mole % of chloride and 1.8 mole % of bromide was prepared by the double jet precipitation method. The average silver halide grain size was 0.4 μm (diameter of a sphere with equivalent volume) and contained Rhodium ions as internal dopant. The emulsion was orthochromatically sensitized by means of compound (I) mentioned above and stabilized by 1-phenyl-5-mercapto-tetrazole.

A base layer coating solution was prepared having the following composition:

    ______________________________________                                         gelatin                  5.5%                                                  carbon black            0.76%                                                  silica particles (5 μm)                                                                              1.6%                                                  ______________________________________                                    

Preparation of the imaging element

The emulsion coating solution and base layer coating solution were simultaneously coated by means of the cascade coating technique to a polyethylene terephthalate support provided with a pack of two backing layers such that the base layer coating was coated directly to the side of the support opposite to the side containing said backing layers. The emulsion layer was coated such that the silver halide coverage expressed as AgNO₃ was 1.5 g/m² and the gelatin content was 1.5 g/m². The emulsion layer further contained 0.15 g/m² of 1-phenyl-4,4'-dimethyl-3-pyrazolidone and 0.25 g/m² of hydroquinone. The base layer was coated such that the amount of gelatin in the coated layer was 3 g/m².

The layer nearest to the support of the backing layer pack contained 0.3 g/m² of gelatin and 0.5 g/m² of the antistatic agent co(tetraallyloxyethane/methacrylate/acrylic acid-K-salt) polymer. The second backing layer contained 4 g/m² of gelatin, 0.15 g/m² of a matting agent consisting of transparent spherical polymeric beads of 3 micron average diameter according to EP 0080225, 0.05 g/m² of hardening agent triacrylformal and 0.021 g/m² of wetting agent F₁₅ C₇ --COONH₄.

The thus obtained element was dried and subjected to a temperature of 40° C. for 5 days and then the emulsion layer was overcoated with a layer containing PdS as physical development nuclei, hydroquinone at 0.4 g/m² and formaldehyde at 100 mg/m².

The following processing solutions were prepared:

    ______________________________________                                         Activator:  A      B      C    D    E    F    G                                ______________________________________                                         sodium hydroxide                                                                           15     15     15   30   30    30  30                               (g)                                                                            sodium sulphite anh.                                                                       35     35     35   35   35    35  35                               (g)                                                                            compound 1 of table                                                                        --     50     250  --   --   250  250                              1 (mg)                                                                         compound 3 of table                                                                        15     15     15    7   15    7   15                               2 (g)                                                                          2-mercapto-5-n.-                                                                           150    150    150  300  300  300  300                              heptyl-oxa-3,4-                                                                diazole (mg)                                                                   water to make                                                                              1 l    1 l    1 l  1 l  1 l  1 l  1 l                              ______________________________________                                         Neutralization solution                                                        ______________________________________                                         Citric acid  10     g                                                          sodium citrate                                                                              35     g                                                          sodium sulphite anh.                                                                        5      g                                                          phenol       50     mg                                                         water to make                                                                               1      l                                                          ______________________________________                                         Dampening solution                                                             ______________________________________                                         water        880    ml                                                         citric acid  6      g                                                          boric acid   8.4    g                                                          sodium sulphate anh.                                                                        25     g                                                          ethyleneglycol                                                                              100    g                                                          colloidal silica                                                                            28     g                                                          ______________________________________                                    

The above described imaging element was image-wise exposed and processed with one of the above described activators, subsequently neutralized at 25° C. with the neutralization solution described above and dried.

The printing plates thus prepared were mounted beside each other on the same offset printing machine (Heidelberg GTO-52) and were printed under identical conditions. During the printing run the described dampening solution was used.

Evaluation

Before printing the reflection density of the plate in the printing areas (D_(p)) and non-printing areas (D_(N)) were measured. The larger the difference between these two values the larger the better a visual evaluation of the printing plate can be made. The printing plates were used to print up to 3000 copies and the tone values of the 20% raster and 40% raster were measured on copy number 100 and 3000. The relative decrease of these tone values which is a measure for the printing endurance of the printing plate was then calculated for each of the printing plates. A tone value decrease (R₂₀) for the 20% raster and one for the 40% raster is thus obtained (R₄₀). The results are shown in table 3.

                  TABLE 3                                                          ______________________________________                                         Transfer developer                                                                             R.sub.20                                                                              R.sub.40 D.sub.P                                                                             D.sub.N                                   ______________________________________                                         A               80     18       0.45 1.03                                      B               37     18       0.46 1.08                                      C               30     15       0.46 1.16                                      D               91     59       0.54 1.23                                      E               39     21       0.46 1.16                                      F               38     20       0.53 1.25                                      G               26     12       0.47 1.21                                      ______________________________________                                    

From the above table it is seen that the decrease of the tone values of the 40% raster and especially the 20% raster which is much more susceptible to wearing of the printing plate is reduced in the presence of the meso-ionic compound. It can further be seen that the contrast of the image on the plate is in most cases improved in the presence of said compound.

EXAMPLE 2

The following processing solutions were prepared:

    ______________________________________                                         Activator                L      M                                              ______________________________________                                         potassium hydroxide (g)  35     35                                             sodium sulphite anh. (g) 35     35                                             compound 1 of table 1 (mg)                                                                               0     250                                            2-Aminoethyl-aminoethanol (g)                                                                           10     10                                             2-mercapto-5-n.heptyl-oxa-3,4-diazole (mg)                                                              300    300                                            water to make            1 l    1 l                                            ______________________________________                                    

Printing plates were prepared as described in example 1 except that activator L or M were used instead of one of the activators of example 1.

The printing plates thus obtained were mounted side by side on the same offset printing press (Heidelberg GTO 46) and were printed under identical printing conditions. The above mentioned dampening solution was used at a 5% concentration for fountain solution, and K+E 125 ink was used. A compressible rubber blanket was used. Up to 4000 copies were printed.

    ______________________________________                                         Results:                                                                       Activator  Number of acceptable copies                                         ______________________________________                                         L            0                                                                 M          4000                                                                ______________________________________                                    

It can be seen from the above results that the sole addition of compound 1 of table 1 makes it possible to process printing plates in the transfer developer described.

EXAMPLE 3

A similar activator solution as activator G in example 1 was prepared except that instead of compound 1 of table 1 compound 3 of table 1 was used (allyltriazoliumthiolaat) at a concentration of 250 mg/l (activator solution N).

Printing plates were prepared as in example 1 using activators E (example 1) and N (described above). The thus obtained printing plates were then mounted consecutively on an ABDick 360 printing press, and up to 3000 copies were printed. K+E 123 W ink was used and the above mentioned dampening solution was used at a concentration of 5% in a Varn dampening system. A compressible rubber blanket was used.

    ______________________________________                                         Results:                                                                       Activator   R.sub.20                                                                              R.sub.40   D.sub.P                                                                             D.sub.N                                     ______________________________________                                         E           27%    8%         0.47 1.14                                        N           11%    7%         0.45 1.18                                        ______________________________________                                    

From the above presented results it is clear that addition of compound 3 from table 1 to the activator also improves print endurance (lowering of the tone value decrease) and improves visual plate contrast.

EXAMPLE 4

The following activator solutions were prepared:

    ______________________________________                                         Activator:               P      Q                                              ______________________________________                                         EDTA (g)                 1      1                                              sodium hydroxide (g)     30     30                                             sodium sulphite anh. (g) 35     35                                             compound 1 of table 1 (g)                                                                               0.5    0.5                                            2-mercapto-5-n.heptyl-oxa-3,4-diazole (mg)                                                              250    250                                            2-aminoethyl-aminoethanol (g)                                                                           20     20                                             compound 3 of table 2 (g)                                                                               --     1.0                                            water to make            1 l    1 l                                            ______________________________________                                          EDTA = ethylenediaminetetraacetic acid tetrasodium salt                  

Printing plates were prepared as in example 1 with the exception that activators P and Q were used. The thus obtained printing plates were then mounted consecutively on a Heidelberg GTO 46 printing press, and up to 10.000 copies were printed. K+E 125 W ink was used and the above mentioned dampening solution was used at a concentration of 5% in a Varn dampening system. A compressible rubber blanket was used. The tone value decrease for the 20% dot of a 40 lpi screen between copy no. 100 and 10.000 was 10% when activator P was used and 5% in case activator Q was used. 

We claim:
 1. A method for making a lithographic printing plate according to the silver salt diffusion transfer process comprising the steps of image-wise exposing an imaging element comprising on a support a photosensitive layer comprising a silver halide emulsion and an image receiving layer containing physical development nuclei and developing said imaging element in the presence of developing agent(s) using an alkaline processing liquid comprising a meso-ionic triazolium thiolate compound and an alkanolamine.
 2. A method according to claim 1 wherein said meso-ionic thiolate compound corresponds to the following formula: ##STR15## wherein R⁷ and R⁸ each independently represents an alkyl group, alkenyl group, cycloalkyl group, aralkyl group, aryl group or heterocyclic group, A represents an alkyl group, alkenyl group, cycloalkyl group, aralkyl group, aryl group, heterocyclic group or --NR⁹ R¹⁰ and R⁹ and R¹⁰ each independently represents hydrogen, an alkyl group or aryl group and wherein R⁷ and R⁸ or R⁸ and A or R⁹ and R¹⁰ can combine with each other to form a 5- or 6-membered ring.
 3. A method according to claim 1 wherein said alkaline processing liquid further comprises a hydrophobizing agent.
 4. A method according to claim 1 wherein said alkaline processing liquid further comprises a thioether compound. 